SBC Timing

[50 F1]

First off I will admit that I am not the best at this. And when I do get a little better at it I don't work on  a motor for years and I forget everything again.  
What am I doing wrong?  
 
Motor - Mid 70's 350 Chevy, nothing special  
 
Condition- compression, lowest cylinder 145 highest 155  
 
Distributor- Summit knock off of a GM HEI  
 
Valve train- everything working as it should, doesn't seem to be any cam problems, but I didn't measure lift on any of the intakes or exhausts.  
 
Problem:  
 
I can get the motor running perfect but when I do, the vacuum advance canister is either up against the valve cover or to close to the carb. And when I move the distributor one tooth it goes all the way over to the other side again, either to close to the cover or to close to the carb.  
 
Am I moving the distributor more than one tooth thinking I am only moving it one tooth?  
 
The other night I had the distributor out 5 times.  Pull #1 plug, put finger in plug hole roll motor over tell finger blows out. With  #1marked  on the distributor body. Put distributor in with the rotor lined up with the mark on distributor body. Put it all back together. Start motor, pop and snort rotate distributor towards valve cover smooths out runs great, starts great. vacuum canister up against valve cover.  
 
When I do the finger in plug hole, when it blows out I look at the timing mark I am close to 0 on timing tab. But when I put the timing light on it when it is running good it is way off. I have a timing light that you can push a button and it will tell you how far advanced you are. It is 35 degrees at idle (800rpms) with the vacuum line from the carb plugged and 65 degrees at 2000 rpms. I know it should be around 12 and 32  

I bought a car with this motor in it years ago. It has a after market cheapie timing tab on it. I know people have had problems with those.
 
What am I doing wrong?  
 
Thanks for the help  
Mike

[chimp koose]

The timing marks are different for different balancers , requiring a different timing tab . Maybe the tab you have does not match the balancer you are using. You could look at the block numbers and figure out when it was made and then you might know which timing tab it should have . Also , if the timing puts the vacuum can against the valve cover , you could always just shift the plug wires over by one terminal and see if that puts the cannister in a better spot  when you have the timing set .

[sirstude]

Slowly turn the engine by hand with a stiff wire in the #1 plug hole until you find top dead center.  Then take a look at the timing tab and see if it matches at 0 degrees.  Lots of issues can cause it to be off including the balancer shifting on the rubber ring.  You could also be moving a couple of teeth like you asked.  Be careful when turning motor so you don't get the wire wedged.

[Rochie]

I have posted this a number of times. It explains timing and vacuum advance as well as I have ever seen it. Keep in mind this is for gm's, ford and some Chryslers used ported vacuum.
Timing 101
This was written by a former GM engineer as a response to a similar question on a Camaro board:
As many of you are aware, timing and vacuum advance is one of my favorite subjects, as I was involved in the development of some of those systems in my GM days an d I understand it. Many people don't, as there has been very little writt en about it anywhere that makes sense, and as a result, a lot of folks a re under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat d own the other day and wrote up a primer on the subject, with the objectiv e of helping more folks to understand vacuum advance and how it works toge ther with initial timing and centrifugal advance to optimize all-around operation and performance. I have this as a Word document if anyone w ants it sent to them - I've cut-and-pasted it here; it's long, but hopeful ly it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to b urn than rich mixtures; idle in particular, as idle mixture is affected b y exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing mo re burn time so that peak cylinder pressure is reached just after TDC fo r peak efficiency and reduced exhaust gas temperature (wasted combustio n energy). Rich mixtures, on the other hand, burn faster than lean mixt ures, so they need to have "the fire lit" later in the compression cycle (s park timing retarded slightly) so maximum cylinder pressure is still achie ved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distri butor advances spark timing purely as a function of engine rpm (irrespectiv e of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on to p of the autocam mechanism. The amount of advance added by the distributor , combined with initial static timing, is "total timing" (i.e., the 34- 36 degrees at high rpm that most SBC's like). Vacuum advance has absolut ely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum adv ance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximu m cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing sett ing (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes lon ger to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light s et up so you could see the balancer as you were going down the highway, you 'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plat es open, manifold vacuum drops, and the vacuum advance can returns to ze ro, retarding the spark timing back to what is provided by the initial st atic timing plus the centrifugal advance provided by the distributor at th at engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the othe r hand, responds to engine load and rapidly-changing operating conditio ns, providing the correct degree of spark advance at any point in time ba sed on engine load, to deal with both lean and rich mixture conditions. B y today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel eco nomy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttl e conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it' s all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlli ng vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (wher e it was exposed to full manifold vacuum at idle) to above the throttle pl ate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its opti mum value), and these applications also had VERY low initial static timin g (usually 4 degrees or less, and some actually were set at 2 degrees A FTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like c rap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations fo r these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixt ure = low emissions), but it was inoperative at idle, which caused all mann er of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the H arry high-school non-vacuum advance polished billet "whizbang" distributor s you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrif icing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacu um advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines , especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren?t fully-deployed until they see about 15? Hg. Manifold vacuum, so those cans don?t work very well on a modified engine; with less than 15? Hg. at a rough idle, the sto ck can will ?dither? in and out in response to the rapidly-changing mani fold vacuum, constantly varying the amount of vacuum advance, which create s an unstable idle. Modified engines with more cam that generate less than 15? Hg. of vacuum at idle need a vacuum advance can that?s fully-deployed at least 1?, preferably 2? of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $ 10 at NAPA) provides the same amount of advance as the stock can (15 degree s), but is fully-deployed at only 8? of vacuum, so there is no variation in idle timing even with a stout cam.

< B>For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifol d vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it ? they don?t understand it, they're on commission, and they want to sell "ra ce car" parts.
Written by JohnZ,
Washing ton, Michigan